Mitochondrial DNA (mtDNA) molecules of some rats from the SASCO colony contain 5 sites at which the restriction enzyme EcoRI cleaves (designated Alb2 mtDNA) while the mtDNA molecules of other rats (Albl) from this colony contain the same 5 sites plus 2 extra sites. MtDNA molecules from wild Utah rats (WldI) contain 6 of the EcoRI sites found in Albl mtDNA. Albl, Alb2, and WldI mtDNA also differ in their sensitivity to the restriction enzymes HhaI, HaeIII, AluI, HinfI, and HincII. HindIII fragments of the three types of mtDNA, cloned using pBR322 will be used: 1) to sequence nucleotides in corresponding regions of molecules which differ in sensitivity to restriction enzymes, and 2) to determine whether regions which differ in restriction enzyme sensitivity are within or between transcribed regions. Using the products of fusion of tissue culture cells containing A1bl and Alb2 mtDNA, an attempt will be made: 1) to identify and quantitate recombinant mtDNA molecules, and 2) to compare survival of Albl, A1b2 and recombinant mtDNA molecules under the influence of Albl and/or Alb2 nuclei. MtDNA from progeny of crosses between Albl and Alb2 rats will be used to evaluate the males contribution to the mtDNA of this organism.